Ships contribute a significant amount of carbon dioxide to the atmosphere.

The International Maritime Organization (IMO) calculated that ocean-going vessels released 1.12 billion metric tons of carbon dioxide in 2007. This is equivalent to the annual greenhouse gas emissions from over 205 million cars, or more cars than were registered in the entire United States in 2006 (135 million).

Shipping is responsible for over three percent of global anthropogenic carbon dioxide emissions and is growing. Over the last three decades, the shipping industry has grown by an average of five percent per year. The IMO predicts that without introducing measures to reduce emissions from shipping, carbon dioxide emissions from the industry could rise to 1.48 billion metric tons by 2020, equivalent to putting 65 million new cars on the road.

Along with CO₂, ships emit various global warming pollutants, including black carbon (BC), nitrogen oxides (NOx) and nitrous oxide (N₂O). These pollutants all contribute to global climate change either directly, by acting as agents that trap heat in the atmosphere, or indirectly by aiding in the creation of additional greenhouse gases.

Reducing black carbon from ships could slow warming, buying time for further steps to reduce carbon dioxide emissions. Black carbon, more commonly known as soot, is made up of fine particles created by the incomplete combustion of a carbon fuel source such as oil or coal. Aging engines and poor engine maintenance can also contribute to incomplete combustion.

Black carbon is known to be a potent warmer both in the atmosphere and when deposited on snow and ice. Black carbon contributes to warming in two ways – through direct absorption of heat in the top of the atmosphere and by lowering the Earth’s albedo, or reflectivity. Unlike greenhouse gases, black carbon is a solid and not a gas and it warms by absorbing sunlight, rather than absorbing infrared or terrestrial radiation.

Assuming reductions are achieved by other sources as is necessary to limit climate change to two degrees Celsius, unregulated shipping emissions could come to account for 12 to 18 percent of global carbon dioxide emissions in 2050. Oceana urges the U.S. EPA to regulate carbon dioxide emissions from ships, as well as implement technical and operational measures to reduce emissions immediately.